This invention relates to the treatment of phosphoric acid streams recovered from chemical processes, in which such streams contain waste organic materials, particularly sulfur-containing organic materials, to recover the phosphoric acid in usable form, and to eliminate or minimize waste disposal treatment of such acid streams.
One source of phosphoric acid contaminated in such manner is processes for production of aromatic thiols such as thiophenol, ring-substituted thiophenols and naphthalenethiols, by reaction of aromatic sulfonic acids with elemental phosphorus.
Such processes are described, for instance, in U.S. Pat. Nos. 2,947,788 and 3,732,969 of Harold Pitt. In these, an aromatic sulfonic acid such as benzenesulfonic acid is reacted with elemental phosphorus in a first stage to produce an aromatic disulfide and phosphorus acid. In the second stage, the aromatic disulfide is reacted with additional elemental phosphorus and water (with the phosphorous acid usually still present) to produce the aromatic thiol and phosphoric acid. Iodine or iodine-containing materials are employed as catalysts. The aromatic thiol is recovered from the reaction mixture by steam distillation.
The phosphoric acid remaining after steam distillation contains a number of impurities resulting from the overall process. These include organic sulfur compounds such as unreacted aromatic sulfonic acids, aromatic disulfides, aromatic sulfones and other sulfur-containing by-products, and also iodine and/or hydrogen iodide (from the catalyst utilized in the process). In general, the phosphoric acid stream contains at least several percent of such impurities and is not satisfactory for commercial use. It must therefore be disposed of as waste, for instance by deposit in landfills. Such disposal has become undesirable and in addition results in a loss of potentially valuable product.
Another source of such contaminated phosphoric acid lies in processes for production of sulfur-containing organophosphorus acids and esters such as thio-, dithio-, and trithiophosphates, phosphonates, chloridophosphates and phosphonates, etc. These substances, which are useful as intermediates in production of materials such as pesticides, flame retardants, and materials for oil recovery, may be produced by a number of methods such as reaction of various phosphorus sulfides with oxygenated compounds (e.g. alcohols). In such processes, waste streams may be produced, for instance, in distillation, which contain phosphoric acid contaminated with various sulfur-containing substances.
Other sources for such phosphoric acid streams may include processes in which phosphorus compounds such as phosphorus trichloride, phosphorus pentachloride or phosphorus oxychloride (POCl.sub.3) are used as reactants or catalysts.
Recovery of phosphoric acid from such processes may be desirable for economic or environmental reasons.